When the Sun Goes Dark

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A total solar eclipse will be visible across the U.S. on Monday, August 21. Shannon Schmoll, director of the Abrams Planetarium at Michigan State University, explains why and how it happens, and what we can learn from an eclipse.

How do we know when an eclipse is going to happen? How do we know in advance where it will be visible?

Solar eclipses happen when our view of the sun is blocked by the moon. When the moon lines up between the sun and Earth, the moon will cast a shadow onto Earth. This is what we on the ground observe as a solar eclipse.

We know when they’ll happen because over centuries astronomers have measured very precisely the motions of the Earth, moon and sun, including their orbital shapes, how the orbits precess and other parameters.

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